1. Field of the Invention
The present invention relates to a coordinates input device such as a pressure sensitive tablet or the like wherein the position of a pressed point is detected when two spaced apart and oppositely disposed resistive sheets are pressed together by a pen or a finger, and also to a method for such a device.
2. Description of the Related Art
It is generally known to provide a coordinates input device called a tablet as input means for information processing apparatus such as a personal computer or a word processor. Such a tablet is used to input positions, characters, graphics, etc. Various types of tablets have heretofore been proposed, several of which, for example, pressure sensitive type, electromagnetic induction type, electrostatic induction type, etc. have already been put into practical use. Among others, the pressure sensitive type tablet has achieved widespread use because of its simplicity in construction.
FIG. 1 is a circuit diagram showing the basic configuration of a prior art pressure sensitive coordinates input device 1. The coordinates input device 1 comprises: a coordinates input section (pressure sensitive tablet) 2 which is pressed by the operator; an A/D converter 3 which detects the potentials of the electrodes (to be hereinafter described) provided in the coordinates input section 2; and a central processing unit (CPU) 4 which controls the supply voltage for the coordinates input section 2 and which, based on the output from the A/D converter 3, detects the coordinates of a point entered on the coordinates input section 2.
The coordinates input section 2 includes plane resistive sheets 5 and 6 whose surfaces are formed from uniform plane resistive material such as indium oxide. Along the opposite edges of the plane resistive sheets 5 and 6, there are provided electrodes XLa, XHa and YLa, YHa which are formed from silver paste or the like. The plane resistive sheets 5 and 6 are disposed so that the electrodes of one of the plane resistive sheets extend at right angles with the electrodes of the one of the plane resistive sheets.
The resistance of the plane resistive sheets 5 and 6 are generally in the order of several hundred ohms, the resistance between the electrodes XLa and XHa being designated by Rx and the resistance between the electrodes YLa and YHz by Ry.
When the plane resistive sheet 6, which serves as an input plane of the coordinates input section 2, is not touched with a pen or a finger, the two plane resistive sheets 5 and 6 are slightly separated by a spacer or the like, thus keeping the plane resistances of the plane resistive sheets 5 and 6 from contacting each other.
Switching elements SxL, SxH, SyL, and SyH are connected to the electrodes XLa, XHa, YLa, and YHa, respectively. The CPU 4 controls these switching elements to switch the electrodes XLa, YLa between a low level (ground potential) and a high impedance state and the electrodes XHa, YHa between a high level (+5 V in this example) and a high impedance state.
With this configuration, it is possible not only to put the plane resistance of the plane resistive sheet 5 in a high impedance state by putting the electrodes XLa and XHa both in a high impedance state, but also to provide a gradient potential to the plane resistance by setting the electrode XLa low and XHa high. This also applies to the electrodes YLa and YHa.
Let us now consider the situation where the plane resistive sheets 5 and 6 of the coordinates input section 2 are brought into contact with each other at a pressing point Pi. In this situation, when the switching elements SxL and SxH are both turned on to set the electrode XLa low (at ground potential) and XHa high (at +5 V) and the switching elements SyL and SyH are both turned off to put the corresponding electrodes in the high impedance state, the potential at the electrode YLa represents the potential at the contact point Pi. Under these circumstances, when the resistance from the electrode XLa to the contact point Pi is denoted as Rx1 and the resistance from the contact point Pi to the electrode XHa as Rx2, the potential at the contact point Pi represents the potential at which 5 V is divided in the ratio of Rx1:Rx2. The x coordinate of the contact point Pi can thus be computed.
Next, when the switching elements SyL and SyH are both turned on to set the electrode YLa low (at ground potential) and YHa to high (at +5 V) and the switching elements SxL and SxH are both turned off to put the corresponding electrodes in the high impedance state, the potential at the electrode XLa represents the potential at the contact point Pi. When the resistance from the electrode YLa to the contact point Pi is denoted as Ry1 and the resistance from the contact point Pi to the electrode YHa as Ry2, the potential at the contact point Pi represents the potential at which 5 V is divided in the ratio of Ry1:Ry2. The y coordinate of the contact point Pi can thus be computed.
The potentials at the electrodes XLa and YLa are detected by the A/D converter 3 having a relatively high input impedance, the potentials then being converted to digital data and supplied to the CPU 4. The CPU 4 thus computes the x and y coordinates of the contact point Pi on the basis of the potentials of the electrodes XLa and YLa.
In the above coordinates input device 1, the reference voltage VREF for the A/D converter 3 is set at a fixed level of +5 V which is equal to the supply voltage for the electrodes XHa and YHa of the coordinates input section 2.
Generally, in a pressure sensitive tablet, even when the same point is pressed, the resulting resistances slightly vary depending on the condition of the pressure. The resistance variations result in variations in the voltages VINX and VINY input to the A/D converter 3. The voltage VINX is a voltage measured along the x-axis and the voltage VINY a voltage measured along the y-axis.
Since the A/D converter 3 performs conversion based on the fixed reference voltage VREF, errors are inevitably contained in data DI0-DI7. Such low-resistance variations are a problem inherent in the coordinates input section 2, and it is not possible to reduce or completely eliminate the variations.
To overcome this problem, a method is proposed wherein the measured voltages are corrected by software processing by the CPU 4. In this method, when pressure is applied to the coordinates input section 2, first the switching element SxH alone is turned on to supply 5 V to the electrode XHa. This sets the electrode YLa to a high level, the potential of which is measured to provide an x-axis correction voltage VINX1. Therefore, the x-axis correction value XSET is obtained by the following equation 1. EQU XSET=VINX1/5 (V) [Equation 1]
Then, the x-axis corrected voltage VINXf is obtained by the following equation 2. EQU VINXf=XSET.times.VINX [Equation 2]
Next, the switching element SyH alone is turned on to set the electrode YHa at 5 V. This sets the electrode XLa to a high level, the potential of which is measured to provide a y-axis correction voltage VINY1. Therefore, the y-axis correction value YSET is obtained by the following equation 3. EQU YSET=VINY1/5 (V) [Equation 3]
Then, the y-axis corrected voltage VINYf is obtained by the following equation 4. EQU VINYf=YSET.times.VINY [Equation 4]
However, according to such correction by software, since corrections are made for both x- and y-axis for every data sampling, an appreciable time is spent in calculation, and therefore, this method is not effective unless the operating speed of the CPU 4 is fast enough.